This invention relates to a multi-axis capacitive transducer and a manufacturing method for producing multi-axis capacitive sensors and actuators.
Micro-Electro-Mechanical-Systems (MEMS) comprise the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate through micro-fabrication processes. In the following context the term transducer is in a summarized manner used for capacitive sensors and for capacitive actuators.
The realisation of reliable force sensing during manipulation of micro-objects is an important objective of current research activities. At present, the most common technique used for force sensing in micromanipulation is that of strain gauges. Nowadays, micromanipulations are performed using either mobile micro-robots or a precise positioning device under control of an optical or scanning electron microscope. During the initial state of development, engineers have concentrated on the design of different micro-handling tools such as micro-grippers. Often sensor feedback is only given by the means of optical measurement, thus leading to a lack of information about the interaction forces between the end-effectors and the micro-components. In order to avoid breaking or damaging objects during the manipulation processes, force feedback is important for a proper functionality. To provide multi-axis force information is a requirement for complicated micromanipulation tasks.
Micro-machined accelerometers have been successful as commercial products. They are used for sophisticated control systems in airplanes and advanced automobiles. Many research has been done on a wide variety of sensing mechanisms, among them capacitive measurement. For new accelerometer technology to be attractive it must be low cost, reliable and perform well. Nowadays, no multi-axis accelerometers featuring 6 degrees of freedom measurement are commercially available.
Most multi-degrees of freedom sensors that have been developed in the last years are based on the piezo-electric effect. Only few works has been done on capacitive multi-degree of freedom sensors. The difference between state of the art capacitive multi-degrees of freedom sensors and the new sensor design presented here are discussed.
In <<A bulk micro fabricated multi-axis capacitive cellular force sensor using transverse comb drives>>, Sun Yu et al., Journal of Micromechanics and Microengineering, Vol. 12, 2002, pages 832-840, the design of a 2-DOF capacitive force sensor is presented. The disclosed sensor allows a measurement of forces only in the wafer plane Fx and Fy. Forces Fz perpendicular to the wafer plane and moments are not measurable.
A multi-axis micro accelerometer is presented in <<A 3-axis force balanced accelerometer using a single proof-mass>>, Mark A. Lemkin et al., Transducers 1997, International Conference on Solid-State Sensors and Actuators, Chicano, June 16-19, 1997, pages 1185-1188. Accelerations out of the wafer plane are measured by a change of overlapping area, which has a relatively small sensitivity. No capacitor plates in the xy-plane are used to measure displacements in z-direction.
The paper <<Detector for force/acceleration/magnetism with respect to components in multi-dimensional directions>>, U.S. Pat. No. 5,437,196, discloses a sensor design which is not suitable for micro-fabrication. Electrodes are attached to the sidewalls of the fixed part and the movable body of the sensor, but they are not part of the mechanical structure.